This invention relates to hydraulic systems of the type requiring relatively large, but variable, flow rates from a hydraulic pump to a hydraulic motor.
In hydraulic systems having hydraulic motors, fairly high flow rates may be encountered. Control over the flow rates is provided by a control valve and where variable flow rates are encountered, the control valve will typically be in the form of a main spool valve.
Spool valves are utilized because of their excellent ability to provide relatively fine control over fluid flow at any of a variety of differing rates. However, when used in systems having relatively large flow rates, the valve must necessarily be of large size. And, it is well known in the art that the leakage encountered in a spool valve increases at a considerable rate as the size of the valve increases.
Thus, when used to control movement of hydraulic cylinders or rotary hydraulic motors, in systems whereat a so-called "neutral" condition is desired, the loading on the hydraulic motor, when the spool valve is commanding a neutral condition, may force hydraulic fluid through the leakage path inherently present in the spool valve. Consequently, the load will tend to creep.
As a result of this difficulty, typical hydraulic systems featuring large spool valves are also provided with a check valve interposed between the spool valve and the hydraulic motor which will allow relatively free flow of fluid from a pump through the main spool valve to the motor to cause the same to change its position. Conversely, when the spool is not set to direct fluid to the cylinder, the check valve, which frequently will be a poppet type valve or other valve having very low leakage, will close to prevent creeping when the spool is shifted to a neutral condition.
At the same time, in many hydraulic systems, there is the possibility that a so-called negative load condition may come into existence. This will occur when the load is acting in concert with the application of fluid to the motor and/or the relief of fluid from the motor. Because the two forces are acting in the same direction, there may result a more rapid movement of the load than is desired.
To solve this difficulty, the prior art has resorted to the provision of so-called flow control valves which control the rate of exit of hydraulic fluid from a hydraulic motor. As a negative load condition increases in severity, the flow control valve will begin to close, thereby tending to retard the rate of fluid relief from the motor to slow down its movement and that of the load controlled thereby. And, in the usual case, a further valve will be provided for the purpose of controlling the flow control valve.
As a result, in a typical system of the type described, at least four valves are employed solely to provide the type of control mentioned previously in connection with fluid flow from but a single port of a hydraulic motor. Needless to say, considerable expense is involved. Moreover, in many instances, the space required by the number of valves may be somewhat greater than that available at the place of installation of the system.